1. Field of the Invention
The present invention relates to a lithium-ion battery pack, and more particularly to a lithium-ion battery pack provided with a cell balance system for keeping uniform the amount of electric charge accumulated in individual battery cells connected in series.
2. Description of the Prior Art
A lithium-ion battery pack, which includes a plurality of lithium-ion battery cells connected in series, is rechargeable, meaning that a battery pack discharged by use can be recharged for further use. To achieve safe recharging, it is essential to prevent overcharging of the battery cells, and thus the charging of the battery cells is typically so controlled as to be stopped as soon as the voltage across any one of the battery cells reaches the maximum permissible voltage.
However, since the individual battery cells discharge different amounts of electric charge in actual use, they often come to have different voltages from one another. In such a case, recharging tends to bring into a fully charged state only the battery cell that has discharged the least during use, leaving the other battery cells in an incompletely charged state.
For this reason, a lithium-ion battery pack is usually provided with a cell balance system, which serves to prevent overcharging and simultaneously achieve uniform charging of all of its battery cells. FIG. 6 shows the circuit configuration of a conventional lithium-ion battery pack.
In this circuit configuration, for each of a number n of battery cells LC(k) (k=1, . . . , n), a cell balance circuit CLB(k) is provided that is composed of a resistor Rb(k), a transistor Tb(k) acting as a switching device, and a control circuit CLC(k). For each battery cell LC(k), the resistor Rb(k) and the transistor Tb(k) are connected in series between the two terminals of the battery cell LC(k). The control circuit CLC(k) monitors the voltage across the battery cell LC(k) to control the conduction state of the transistor Tb(k) in accordance with that voltage.
FIG. 7 shows the circuit configuration of the control circuit CLC(k). The control circuit CLC(k) is composed of a reference voltage generator VG(k), a comparator CMP(k), two input terminals Vp(k) and Vn(k) connected to the positive and negative terminals, respectively, of the battery cell LC(k), and an output terminal Vc(k) connected to the gate of the transistor Tb(k). The comparator CMP(k) has two input terminals of which one is connected to the input terminal Vp(k) and the other is connected through the reference voltage generator VG(k) to the input terminal Vn(k), and has an output terminal that is connected to the output terminal Vc(k).
The reference voltage generator VG(k) receives, via the input terminal Vn(k), the voltage at the negative terminal of the battery cell LC(k), and outputs a reference voltage that is equal to a predetermined level V.sub.TH0 added to the received voltage. The comparator CMP(k) receives, via the input terminal Vp(k), the voltage at the positive terminal of the battery cell LC(k), and compares the received voltage with the voltage fed from the reference voltage generator VG(k). In accordance with which of these two voltages is higher, the comparator CMP(k) outputs a first or a second predetermined voltage in such a way as to make the transistor Tb(k) conduct when the voltage at the positive terminal of the battery cell LC(k) is higher than the reference voltage. When the transistor Tb(k) conducts, a bypass path is formed through the resistor Rb(k) in parallel with the battery cell LC(k).
FIG. 8 shows the relationship between the voltage of the battery cell LC(k) and the operation of the transistor Tb(k) during charging. In FIG. 8, the graph at (a) shows the voltage (cell voltage) V.sub.cell between the two terminals of the battery cell LC(k), and the graph at (b) shows whether the transistor Tb(k) is in a conducting (on) state or in a non-conducting (off) state. In both graphs, the lapse of time after the start of charging is taken along the horizontal axis.
As the result of the operation of the control circuit CLC(k) as described above, when the cell voltage V.sub.cell is lower than the predetermined level V.sub.TH0, the transistor Tb(k) is kept in the non-conducting state, with the result that a current flows through the battery cell LC(k) so as to charge it. By contrast, when the cell voltage V.sub.cell is equal to or higher than the predetermined level V.sub.TH0, the transistor Tb(k) is kept in the conducting state, with the result that the current flows mostly through the bypass path, greatly slowing down the progress of charging.
Consequently, the battery cells that have a cell voltage lower than the predetermined level V.sub.TH0 are charged with priority. On the other hand, for the battery cells that have a cell voltage equal to or higher than the predetermined level V.sub.TH0, the increasing rate of their cell voltage is kept so low that the maximum permissible voltage V.sub.OCH that is set to prevent overcharging is reached slowly. In this way, the amount of electric charge accumulated in the individual battery cells is made substantially uniform.
However, in this conventional cell balance system, the control circuit CLC(k) controls the operation of the transistor Tb(k) on the basis of only one predetermined level V.sub.TH0. As a result, the adjustment of the charge amount needs to be started as late as immediately before the completion of charging, and thus an unduly long time is required to make the charge amount of all of the battery cells uniform. Moreover, for the battery cells whose cell voltage has already reached the predetermined level V.sub.TH0, a current is kept flowing through the bypass path, and thus the resistor Rb(k) produces much heat. To prevent the resistor Rb(k) from being destroyed by heat, it is inevitable to give it a considerably high resistance to reduce the current that flows through the bypass path. This, however, makes even longer the time required to make the charge amount of the individual battery cells uniform.
Japanese Laid-Open Patent Application No. H8-19188 proposes a charging apparatus for charging a set battery having a plurality of battery cells connected in series. In this charging apparatus, for each of the battery cells, a bypass path is provided that is composed of a resistor and a transistor connected in series, and in addition a control circuit is provided that controls the conduction state of each bypass path individually. During charging, the control circuit monitors the cell voltage of each battery cell and calculates the voltage difference .DELTA.Vmin between the cell voltage of each battery cell and the minimum cell voltage; in accordance with this voltage difference .DELTA.Vmin, the control circuit controls the conduction state of the bypass path by one of the following two methods.
According to the first method, for the battery cells whose voltage difference .DELTA.Vmin is equal to or higher than a first predetermined level, the bypass path is made to conduct so that the difference in the charge amount among the battery cells will be reduced. Moreover, when the voltage difference .DELTA.Vmin of the battery cells for which the bypass path has been kept conducting to keep the charging rate low becomes equal to a second predetermined level that is lower than the first predetermined level, the bypass path for those battery cells is cut off so that charging will be continued while maintaining the reduced difference in the charge amount.
According to the second method, the current that flows through the bypass path is adjusted in accordance with the voltage difference .DELTA.Vmin. Specifically, for each battery cell, the transistor is so controlled as to operate with a duty factor that varies in proportion to the voltage difference .DELTA.Vmin. The higher the duty factor with which the transistor of a bypass circuit operates, the higher the current that flows through the bypass circuit. Thus, the larger the amount of electric charge accumulated in a battery cell, the lower the charging rate at which the battery cell is charged. In this way, the difference in the charge amount among the battery cells is gradually reduced.
On the other hand, Japanese Laid-Open Patent Application H9-28042 proposes a charge control apparatus for charging a set battery. In this charge control apparatus, too, for each of the battery cells, a bypass path is provided that is composed of a resistor and a transistor connected in series, and in addition a control circuit is provided that controls the conduction state of each bypass path individually. Before starting charging, the control circuit monitors the cell voltage of each battery cell and calculates the voltage difference .DELTA.Vmax between the cell voltage of each battery cell and the maximum cell voltage; in accordance with this voltage difference .DELTA.Vmax, the control circuit controls the conduction state of the bypass path by one of the following three methods.
According to the first method, for the battery cells whose cell voltage has reached a predetermined level, the bypass path is made to conduct; in addition, the current that is allowed to flow through each bypass path is determined in accordance with the voltage difference .DELTA.Vmax so that the cell voltage of all of the battery cells will reach, substantially at the same time, the maximum permissible voltage that is set to prevent overcharging. According to the second method, for each battery cell, the level of the cell voltage that activates the transistor of the bypass path is determined in accordance with the voltage difference .DELTA.Vmax so that the cell voltage of all of the battery cells will reach, substantially at the same time, the maximum permissible voltage that is set to prevent overcharging. According to the third method, the variation of the cell voltage among all of the battery cells is calculated from the voltage difference .DELTA.Vmax of each battery cell so that the bypass path will be allowed to conduct only when the variation exceeds a predetermined range.
The charging apparatus and the charge control apparatus described above are both capable of adjusting the current that flows through the bypass path, and are therefore capable of making the charge amount of all of the battery cells uniform with ease. It is possible even to apply the control methods used in those apparatuses to a cell balance system for a lithium-ion battery pack.
However, in both of the apparatuses described above, the bypass path is controlled on the basis of the voltage difference .DELTA.Vmin between the cell voltage of each battery cell and the minimum cell voltage, or the voltage difference .DELTA.Vmax between the cell voltage of each battery cell and the maximum cell voltage. This requires, in addition to circuits for monitoring the cell voltage of the individual battery cells, circuits for performing comparison to find the minimum or maximum cell voltage, and also circuits, or a computing device such as a microcomputer, for calculating voltage differences. Thus, it is inevitable to use complicated and large-scale control circuits, in particular in a control module that is composed solely of analog circuits without using any logic circuit such as a microcomputer. Using such control circuits in a cell balance system for use in a battery pack ends in making the entire battery pack unduly large.
A wide variety of lithium-ion battery packs are commercially available, ranging from small-scale ones that include as few as two battery cells to large-scale ones that include a hundred or more battery cells. The smaller the scale of a lithium-ion battery pack, the more seriously the compactness of the battery pack is spoilt by the introduction of a large-scale cell balance system. Since small-scale battery packs are used mainly in portable electronic appliances such as portable telephones, they are required to be as small and light as possible, and accordingly a cell balance system for use in such appliances is required to have as simple a circuit configuration as possible. For this reason, it is not necessarily advisable to apply the control methods used in the two apparatuses described above to a cell balance system for use in a lithium-ion battery pack.